1. Field of the Invention
The present invention relates to an acceleration sensor and, more particularly, to an acceleration sensor suited to detect a large change in velocity which is caused by a collision of a vehicle.
2. Description of the Prior Arts
This type of acceleration sensor disclosed in U.S. Pat. No. 4,827,091 comprises: a cylinder composed of a conductive material; a magnetized inertial member so mounted in the interior of the cylinder as to be movable in the longitudinal direction of the cylinder; a conductive member mounted on at least one end surface of the magnetized inertial member and disposed, in the longitudinal direction of the cylinder; a pair of electrodes disposed at one end of the cylinder in the longitudinal direction and made conductive through the conductive member upon contact with the conductive member of the magnetized inertial member; and an attracting member disposed at the other end of the cylinder in the longitudinal direction and composed of a magnetic material which magnetically attracts the magnetized inertial member.
In this acceleration sensor, the magnetized inertial member and the attracting member attract each other. When no or almost no acceleration is applied to the acceleration sensor, the magnetized inertial member is stationary at the other end within the cylinder.
If a relatively large acceleration acts on this acceleration sensor, the magnetized inertial member moves while resisting the attracting force of the attracting member. When the magnetized inertial member is moving, an induced current flows in this cylinder. A magnetic force is given to the magnetized inertial member, thereby biasing this member in a direction opposite to the moving direction to provide a state where the magnetized inertial member is braked. Its moving velocity is thereby reduced.
If the acceleration is smaller than a predetermined value, i.e., threshold value, the magnetized inertial member does not reach the top end of the cylinder. The magnetized inertial member moves and then stops midways. Subsequently, the magnetized inertial member is pulled back to the other end by the attracting force of the attracting member.
Whereas if the acceleration is greater than the predetermined value i.e., threshold value, in other words, if the vehicle mounted with this acceleration sensor is, for instance, collided, the magnetized inertial member reaches one end of the cylinder. A conductive layer on the top end surface of the magnetized inertial member contacts both of the pair of electrodes, thus making the electrodes conductive to each other. If a voltage is applied beforehand between the electrodes, the current flows in between the electrodes when the electrodes are short-circuited. The collision of the vehicle is detected from this current.
In the conventional acceleration sensor, the magnetized inertial member moves in the interior of the cylinder. Hence, the cylinder considerably increases in size in the longitudinal direction, and correspondingly the acceleration sensor as a whole assumes a large and long cylindrical shape. For this reason, an intra-vehicle space for incorporating the acceleration sensor becomes relatively large.